In the prior art, research has been effected on vibration damping rubbers and dampers having electrorheological fluid sealed therein with a view toward developing practically acceptable devices having the excellent function of adjusting spring or loss properties by changing the applied electric field in response to an input. In order that vibration damping rubbers and dampers utilizing electrorheological fluid perform in practice, it is necessary to completely seal the electro-rheological fluid in a container and to develop a fully stable electrorheological fluid in which dispersed fine particles do not settle or coalesce over a long period of time. Because of the differential specific gravity between an insulating oil as the medium and the dispersed fine particles, none of the prior art electro-rheological fluids could meet these requirements.
One known material whose viscoelasticity varies with an electric field applied thereto is a field effect flexible polymer gel. This polymer gel finds a limited application since it can operate only in a special solvent.
Also, a material whose modulus of elasticity varies with an electric field was recently reported by Japanese Patent Application Kokai (JP-A) No. 91541/1991. This variable modulus material is comprised of an electrically insulating polymeric material having dispersed therein a fine powder which will electrically polarize in an electric field. The dispersed fine powder used in this material is hydrophilic fine particles which will polarize in the presence of water, conductive polymer fine particles, or fine particles which do not undergo sufficient electric polarization in an electric field. Thus, some variable modulus materials are difficult to adjust the water content during manufacture and lack the stability of their quality due to their increased temperature dependency. Some allow excessive current flow upon application of an electric field. And some fail to provide a satisfactory change of modulus of elasticity. For these reasons and other, the variable modulus materials are also limited in use.